How complete is complete?

Descriptions of fossils generally include an opening line that comments on how complete the specimen is based on how much material is present. Statements like complete / nearly complete / mostly complete / some elements preserved etc. are all common. These are of course reasonable guesstimates based on what is there, in terms of the total numbers of bones that are present.

However, reasonable though this is, it is of course possible to have very significant numbers of bones of the skeleton be missing and still have an effectively complete specimen. Or to be more precise, to have all of the necessary anatomical information. After all, assuming you have one complete and articulated arm, the other one won’t actually tell you anything you don’t already know (even if it does make for a nicer specimen). You don’t therefore really need one hand and arm or leg and foot or for that matter half the pelvis or shoulder girdle. You can get rid of half the ribs and gastralia and, in theory at least, half of the skull too. In fact given that in most taxa the ribs, gastralia and chevrons are pretty uniform you could loose most of them without compromising any information.

I reckon you could easily loose about 30% of the bones and write as full and accurate description as if you had 100%. Obviously this is just a thought experiment, but it (might) show that a ‘nearly complete’ specimen that’s missing the skull, or both feet might not actually have as much information as an ‘only partially complete’ specimen.

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5 Responses to “How complete is complete?”

Some animals you need very little. Hadrosaurs and ceratopsians for example. All you need is the head- the rest of the body can be based on other specimens in the same family or subfamily with extreme confidence. A Styracosaurus head put on a Centrosaurus body would be unlikely to raise any issues if you said nothing to the researcher. We have had students come to the Royal Tyrrell Museum in the past trying to find differences in postcranial bones among contemporaneous hadrosaurs and ceratopsians and cannot find much. In hadrosaurus, there are some differences in the humerus, ilium, pubis, ischium, sacrum and caudal neural spines, some of these are only subfamily differences. In ceratopsians, there is a difference in curvature between the ischium in the two subfamilies but that is it.

Has that been published Darren? I know people have long said there is little difference in most of the postcranial elements of these, but I’ve not seen anything that actually tried to test this (not that I’ve looked admittedly).

I’m certain Brenda Chinnery-Allgeier was wanting to this quite some years ago at our museum. I recal her coming here, but was disappointed with the results. There were one or two others long before her, but their names are long forgotten.